Kunming/simple batch fix

[Jiangkm3]

Fixed the bugs in the simple batching case and add tests. Use cargo test --package whir --lib -- whir::tests::test_whir --exact --show-output to run the new batching test.

[Jiangkm3]

There are 3 main contributions:

1. Added a new test for batched polynomial testing make_whir_batch_things in src/whir/mod.rs
2. Changed the protocol of the prover so that instead of chunking the previous merkle answers by fold size, chunk by (num_polys * fold_size) to obtain each merkle answer of each polynomial
3. Added a new compute_folds_batched for the verifier that computes the folds of evaluations of each polynomial and perform the same random linear combination as the prover.

[Jiangkm3]

It turns out that compute_folds_batched is unnecessary, instead one needs to process final_randomness_answers when there are only one round.